When working with a SQL Server database, it’s important to understand how the system handles changes and ensures data consistency. One key aspect of this is the concept of checkpoints and recovery time.
When a new row is inserted or an existing one is updated in your database, the SQL Server Database Engine will keep that change in the buffer pool in memory first, without applying each change directly to the database files for IO performance reasons. These data pages located in the buffer pool and not reflected yet in the database files are called Dirty Pages.
The SQL Server Database Engine uses a special kind of process called a checkpoint to write these dirty pages to the data and log database files periodically. A checkpoint creates a mark that is used by the SQL Server Database Engine to redo any committed transactions and roll back any uncommitted transactions in the event of an unexpected shutdown or crash. This ensures database consistency.
There are four types of checkpoints that the SQL Server Database Engine supports:
- Internal Checkpoints: Issued in response to several server events, such as taking a database backup or performing a clean shutdown of the SQL Server service.
- Manual Checkpoints: Triggered by executing the CHECKPOINT T-SQL command. You can specify the checkpoint duration parameter to control the amount of time the checkpoint takes to complete.
- Automatic Checkpoints: Issued automatically in the background when the number of log records in the buffer pool reaches a certain threshold. The recovery interval option specifies the maximum amount of time required by the SQL Server Database Engine to recover the database after restarting the SQL Server.
- Indirect Checkpoints: Introduced in SQL Server 2012, these checkpoints allow you to configure recovery time at the database level. They provide faster and more accurate recovery time compared to automatic checkpoints.
Starting from SQL Server 2016, the default checkpoint type is the indirect checkpoint. This type offers improved performance and efficiency in terms of data page transfer and I/O spikes.
It’s important to note that the recovery time of a database after a system crash depends on the time required to redo the dirty pages, which is often longer than the recovery interval specified for automatic checkpoints. Therefore, adjusting the recovery interval value may enhance performance, especially if rolling back long transactions takes more than 60 seconds or if frequent checkpoint processes are impacting database performance.
When using indirect checkpoints, it’s crucial to consider the impact on disk system performance, especially if the database has a heavy OLTP workload or disk system bottleneck. Indirect checkpoints write dirty pages to the disk more frequently to meet the configured recovery interval value, increasing the write load for the SQL Server.
In conclusion, understanding SQL Server checkpoints and recovery time is essential for maintaining database consistency and performance. By choosing the appropriate checkpoint type and configuring recovery options, you can ensure efficient data recovery and minimize the impact on system performance.
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